orthostatic hypotension (thing)

Postural hypotension is the medical name for the dizziness and light-headedness that you get from standing up too fast. It's caused by the delay while your heart and blood vessels adjust to your new position, during which there is a shortage of oxygen to the brain. Once the heart speeds up and the blood vessels constrict, your blood pressure should return to normal.

Normally this isn't a problem, but if it happens excessively, doesn't fade quickly, or causes fainting, you should see a doctor.